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                    <h1 class="description center-align post-title">【JVM】从JVM角度解读字节码及本地变量表操作数栈工作原理（第七篇）</h1>
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                <h3 id="一、操作数及操作数栈"><a href="#一、操作数及操作数栈" class="headerlink" title="一、操作数及操作数栈"></a>一、操作数及操作数栈</h3><p>我们都知道，不同架构的计算机有自己独特的机器指令集，然而java虚拟机也不例外，java字节码是java虚拟机所使用的指令集，字节码的结构和java虚拟机基于栈的计算模型是密不可分的。在解释执行过程中，当线程要调用方法，此时java虚拟机会为方法分配栈帧，另外需要开辟一块额外的内存空间作为操作数栈，来存放计算所使用的操作数及指令执行后返回的结果。</p>
<p>具体来说，java虚拟机在执行一条指令之前，java虚拟机规范中要求该指令作用的操作数已被压入操作数栈中；在执行指令时，java虚拟机会将该指令所需要参与的操作数从操作数栈中弹出，之后将指令的结果重新压入栈中。下图为栈帧中局部变量表及操作数栈的分布图：</p>
<p><img src="https://i.loli.net/2020/10/23/DbnuUXvtWx56JRd.png" alt="image-20201023225558535"></p>
<p>以java虚拟机加法指令iadd为例，假设在执行该指令前，栈顶的两个元素分别为int类型的值1和int类型的值2；在执行iadd指令时首先会将参与此条指令执行的操作数弹出栈，经过相加计算得到的int类型的3压入栈中，指令执行接收后，操作数栈中的情况如图：</p>
<p><img src="https://i.loli.net/2020/10/23/TvdDX2CnU5qc9GP.png" alt="image-20201023230257067"></p>
<p>由于iadd指令只需要栈顶的两个元素参与指令执行，因此，对于距离栈顶为2的元素，即图中的问号部分，iadd指令并不关心它是否存在，更不会对其进行修改。</p>
<p>java字节码中，有好几条指令是直接用在操作数栈上的，最常见的便是dup（复制栈顶元素），以及pop (舍弃栈顶元素)；dup指令常用于复制new指令所生成的未经初始化的引用。例如在下面这段代码的foo方法中，当执行new指令时，①java虚拟机将指向一块已经分配但尚未初始化内存的引用压入操作数栈中。</p>
<p><img src="https://i.loli.net/2020/10/23/9YhfnTWFO5lNuLp.png" alt="image-20201023230840676"></p>
<p>②然后，虚拟机需要以这个引用作为调用者，调用其构造函数，也就是上面字节码中的invokespecial指令，要注意，invokespecial指令将消耗操作数栈上的元素(①步骤中压入的引用)，作为构造函数的调用者并将以参数的方式传入构造函数(Object类比较特殊，在new时不需要隐式传入this指针),这就是我们前面说的this指针；因此，我们需要使用dup指令复制一份new指令的结果，并用来调用构造函数，当调用返回之后，操作数栈上仍有原本由new指令生成的应用①，可用于接下来的操作(即用来执行上面代码中偏移量为7的字节码)。</p>
<p>pop指令则常用于舍弃调用指令后返回的结果，例如在下面这段代码的foo方法中，我将调用静态的方法bar，bar方法有返回值，foo方法没有返回值；由于对应的invokestatic指令仍旧会将返回值压入foo方法的操作数栈中，因此java虚拟机需要额外执行pop指令，将放回的结果舍弃掉(代码中我的foo方法没有返回值)</p>
<p><img src="https://i.loli.net/2020/10/23/DPqzcXHG2Zx7N3B.png" alt="image-20201023232102701"></p>
<p>需要注意的是，上述两条指令只能处理非long或非double类的值，这是因为long类型或double类型的值需要占用两个栈单元，当需要这些值时，我们需要同时复制栈顶两个单元的dup2指令，以及弹出栈顶两个单月的pop2指令，因为上面的dup、pop一次只能压入或弹出一个栈单元；除了这些指令之外，不算常见，但是能直接作用于操作数栈的还有swap指令，它用来交换栈顶两个元素的值；在java字节码中，还要一部分指令可以直接将常量加载到操作数栈上，以int类型为例，java虚拟机可以通过iconst指令加载-1至5之间的int值，也可以通过bipush、sipush加载一个字节、两个字节所能代表的int值。</p>
<p>java虚拟机还可以通过ldc加载常量池中的常量，例如ldc #18 表示，将加载常量池的第18项，这些常量包括int类型、long类型、float类型、double类型、String类型以及Class类的的常量</p>
<p><img src="https://i.loli.net/2020/10/23/5A6rQLD2tNkevqu.png" alt="image-20201023233107937"></p>
<p><strong>常数加载指令表：</strong></p>
<p>正常情况下，操作数栈的压入和弹出都是一条条指令完成的，唯一的例外情况是在抛异常时，java虚拟机会清除操作数栈上的所有内容，而后将异常实例引用压入操作数栈中。</p>
<p><strong>局部变量区：</strong></p>
<p>java方法栈帧的另外一个重要组成部分就是局部变量区，字节码程序可以将计算的结果缓存再局部变量区中，实际上，java虚拟机将局部变量区当成一个数组，一次存放this指针(仅非静态方法)，所传入的方法参数，以及方法体字节码中的局部变量；和操作数栈一样，long类型的以及double类型的值将占据两个单元，其余的类型仅占据一个单元</p>
<pre class=" language-java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token keyword">long</span> l<span class="token punctuation">,</span><span class="token keyword">float</span> f<span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token punctuation">{</span>
        <span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token punctuation">{</span>
        String s <span class="token operator">=</span> <span class="token string">"Hello world"</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre>
<p>以上这段代码中的foo方法为例，由于它是一个实例方法，因此局部变量数组的第0个单元存放着this指针，第一个参数为long类型，占用两个数组单元，于是1、2两个单元存放着所传入的long类型参数的值，第二个参数则是float类型，于是数组的第3个单元存放着所传入的float类型数值。</p>
<p><img src="https://i.loli.net/2020/10/24/D3GyhwKcHleTp4t.png" alt="image-20201024092115895"></p>
<p>在方法体的两个代码块中，我们分别定义了两个局部变量i和s,由于两个局部变量分部在不同的代码块中，所以其生命周期没有重合之处，也就是第一个代码块被执行完毕后，局部变量区中的i所占用的单元就会被回收，当执行第二个代码块时候s被添加到局部变量区。<br>        存储在局部变量区的值，通常需要加载至操作数栈中，方能进行计算，得到计算结果后再存储至局部变量数组中，这些加载、存储指令是区分类型的，例如int类型的加载指令为iload,存储指令为istore</p>
<p><img src="https://i.loli.net/2020/10/24/kNovK3L9l4QT1gF.png" alt="image-20201024092716302"></p>
<p><strong>局部变量区访问指令表</strong></p>
<p>局部变量数组的加载、存储指令都需要指明所加载单元的下标，距离来说，aload 0指的是加载第0个单元所存储的引用，在前面示例中的foo方法里，指的便是加载this指针，为了加深在方法执行过程中，操作数栈上的操作数分布情况，我们接下来看一个例子：</p>
<p><img src="https://i.loli.net/2020/10/24/31lTZcOepwb68MK.png" alt="image-20201024093156713"></p>
<p>这里我定义了一个bar方法，它将接收一个int类型的参数，进行一系列计算之后再返回，对应字节码中的stack=2,locals=1代表该方法需要的操作数栈空间为2，局部变量数组空间为1，当调用bar(5)时，每条指令执行前后，局部变量数组空间以及操作数栈的分布如下：</p>
<p><img src="https://i.loli.net/2020/10/24/IVYaDkQEAcwXFlm.png" alt="image-20201024101635135"></p>
<h3 id="二、java字节码简介"><a href="#二、java字节码简介" class="headerlink" title="二、java字节码简介"></a>二、java字节码简介</h3><p>前面已经介绍了加载常量指令、操作数栈专用指令以及局部变量区方法的指令，下面我们来看看其他类别的指令；java相关指令，包括各类具备高层语义的字节码，即new(后跟目标类，生成的类的为初始化对象)，instanceof（后跟目标类，判断栈顶元素是否为目标类/接口的实例，将压入1或者0）、checkcast（后跟目标类，判断栈顶元素是否为目标类/接口的实例，如果不是便抛出异常）、athrow（将栈顶异常抛出），以及monitorenter(为栈顶对象加锁)和monitorexit（为栈顶对象解锁）。</p>
<p>此外，该类型的指令还包含<strong>字段访问指令</strong>，即静态字段访问指令getstatic、putstatic,和实例字段访问指令getfield、putfield，这四条指令均附带用于定位目标字段的信息，单所消耗的操作数栈元素皆不同。</p>
<p><img src="https://i.loli.net/2020/10/24/eb7mFJlTqtyj15s.png" alt="image-20201024100246076"></p>
<p>以putfield为例，在上图中，它将把值v存储至对象obj的目标字段中。</p>
<p><strong>方法调用指令</strong>,包括invokestatic、invokespecial、invokevirtual、invokeinterface以及invokedynamic，这几条字节码我们已经反复提及了，这里就不详细展开了；除了invokedynamic外，其它的方法调用指令所消耗的操作数栈元素是根据调用类型及目标方法描述符来确定的，在进行方法调用之前，程序需要依次亚茹调用者(注意invokestatic不需要)，以及各个参数</p>
<p><img src="https://i.loli.net/2020/10/24/RByN7vagSmjAEWe.png" alt="image-20201024100752599"></p>
<p>以上面的代码为例，当调用foo(2)时，每条指令执行前后，局部变量数组空间及操作数栈的分布如下所示：</p>
<p><img src="https://i.loli.net/2020/10/24/EiQzpySwBPmrYn5.png" alt="image-20201024102525432"></p>
<p>数组相关指令，包括新建基本类型的数组的newarray，新建引用类型数组的anewarray,生成多维数组的multianewarray,以及求数组长度的arraylength,另外，它还包括素组的加载指令，以及存储指令,这些指令是区分类型的，例如，int数组的加载指令为iaload,存储指令为iastore</p>
<p><img src="https://i.loli.net/2020/10/24/neU2LAW1kj7bTQH.png" alt="image-20201024102928262"></p>
<p>​                                                                            图：数组访问指令表</p>
<p><strong>控制流指令</strong>，包括无条件跳转goto，条件跳转指令，tableswitch和lookupswitch，返回指令，其中返回指令是区分类型的，例如，返回int值的指令为ireturn</p>
<p><img src="https://i.loli.net/2020/10/24/XQPqMASyrgW31FR.png" alt="image-20201024103113412"></p>
<p>​                                                                            图：方法返回指令表</p>
<p>返回指令外，其它的控制流指令均附带一个或多个字节码偏移量，代表要调整到的位置，例如下面的abs方法中偏移量为1的条件跳转指令，当栈顶元素小于0时，跳转至偏移量为6的字节码</p>
<p><img src="https://i.loli.net/2020/10/24/nL6J9GQVwk5AbTD.png" alt="image-20201024103527051"></p>
<h3 id="三、总结"><a href="#三、总结" class="headerlink" title="三、总结"></a>三、总结</h3><p>今天主要介绍了java方法的栈帧分为操作数栈和局部变量表，通常来说，程序需要将变量从局部变量区加载至操作数栈中，进行一番计算之后再存储回局部变量区中。</p>
<p>java字节码可以划分为多种类型，例如：价值常量指令、操作数栈专用指令、局部变量区访问指令、java相关指令、方法调用相关指令、数组相关指令、控制流指令、计算相关指令。</p>

                
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